How this calculator works
At a fixed height, weight changes in direct proportion to BMI. Enter the target as a BMI, body weight or BMI Prime, then compare it with the current measurement. BMI Prime here is simply BMI divided by the conventional adult boundary of 25. Negative changes mean a lower target; positive changes mean a higher target. This comparison does not select an appropriate goal, calculate a calorie deficit or model how quickly weight would change. It is for adults, outside pregnancy; adult BMI also needs care in athletes and people with unusual muscle mass. The result is an estimate for information only and is not medical advice.
Formula
Current BMI = current kg / height in metres². Target kg = target BMI x height². BMI Prime = BMI/25. Signed change = target kg - current kg. Percentage change = signed change / current kg x 100.
Worked example
At 1.80 m and 90 kg, BMI is 27.78. A target BMI of 25 gives 81 kg: a change of -9 kg or -10%. At 1.60 m, a target weight of 64 kg gives BMI 25 and BMI Prime 1.
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Frequently asked questions
Is the default target a recommendation?
No. It demonstrates the equation. An appropriate goal depends on clinical context, age, body composition and personal circumstances.
Why are adult percentiles absent?
Percentiles depend on a chosen survey population, sex and age, and are not intrinsic to the BMI formula. This page does not invent a population distribution.
Can it estimate how many weeks weight loss takes?
No. A BMI difference does not determine energy needs, body composition change or a safe rate of weight change.